1. Field of the Invention
The present invention relates to an optical information recording medium that allows information signals to be recorded/reproduced with high quality by irradiating a thin film formed on a substrate with a high energy beam such as a laser beam, a method for producing the same, and a method and an apparatus for recording/reproducing information thereon.
2. Description of the Prior Art
Conventionally, it is known that it is possible to cause a phase change between an amorphous phase and a crystalline phase, which have different optical constants (refractive index n and extinction coefficient k), by irradiating a thin film made of a chalcogen material formed on a substrate with a laser light beam for local heating under different irradiation conditions. Utilizing this phenomenon, a so-called phase changeable optical information recording medium has been under development.
In the phase changeable optical information recording medium, a new signal can be recorded while erasing an existing recorded signal, using only a single laser beam, by modulating the laser output between two levels of a recording level and an erasing level in accordance with the information signal and irradiating an information track with the single laser beam. This method is advantageous for recording information for the following reasons: A magnetic circuit component as required by optical magnetic recording is not required, so that the head can be simplified; and erasure and recording can be performed simultaneously, so that a period of time required for rewriting can be shortened.
In such an optical information recording medium, the following structure is common. Dielectric layers having excellent heat resistance are provided as protective layers above and below a recording layer for the purpose of preventing the evaporation of the recording layer and the thermal deformation of the substrate that might occur during repeated use. Furthermore, a reflective layer made of a metal material is provided on the protective layer on the side opposite to the substrate for the purpose of efficiently using incident light and increasing the cooling rate so as to facilitate a change to an amorphous state. Thus, in general, at least four thin films are laminated to form the optical information recording medium.
In order to produce a phase changeable optical information recording medium with high density and large capacity, the following attempts are commonly carried out: forming a smaller mark by using a shorter wavelength of the light source or a higher NA (numerical aperture) of the object lens used for recording, and thus improving the linear density in the circumferential direction and the track density in the radial direction of the recorded mark on the substrate. Furthermore, mark edge recording in which information is defined by the length of the mark has been proposed to improve the linear density, and land and groove recording in which information is recorded both on grooves for guiding laser light formed on the substrate and lands between the guide grooves has been proposed to improve the track density, and both recording methods are adopted.
Furthermore, it was proposed that a plurality of such recordable information layers are laminated via separating layers so that the capacity is increased (e.g., JP 9-212917 A, JP10-505188 A). Moreover, layer recognition means and layer switching means for selecting one of these information layers for recording and reproduction were proposed (e.g. JP10-505188 A).
Improving not only high density but also data processing rates, namely, the velocity of recording/reproducing information, is important. For this reason, improving the linear velocity by rotating a disk at a higher revolution per minute with the radius position unchanged for recording and reproduction is under research.
In such an optical information recording medium, there are some challenges for achieving good and practical signal quality and recording/reproducing characteristics, and some measures have been taken.
First, to obtain large signal amplitudes and high C/N ratios, a large change in the reflectance between crystal and amorphous portions is required. To cause a large change, it is necessary that the absolute value of the difference in the reflectance xcex94R=Rcxe2x88x92Ra is large, where Rc is the reflectance when the recording medium is crystalline, and Ra is the reflectance when the recording medium is amorphous with respect to a laser light beam of wavelength xcex.
In the case of overwriting with a single beam, there is a difference in the end-point temperatures between the case where the base layer before recording is amorphous and the case where it is crystalline even if irradiated with beams of the same power level, because the crystalline portion requires a latent heat of fusion. Therefore, when overwriting a signal, the shape of the mark is distorted by the influence of a signal that has been recorded before the overwriting. This mark distortion causes a drop of the erasure ratio or an increase of errors (jitters) in the time axis direction of reproducing signals. The problem caused by this phenomenon becomes more serious as higher linear velocity and higher density are achieved (e.g., Noboru Yamada, xe2x80x9cPotential of Gexe2x80x94Sbxe2x80x94Te Phase-Change Optical Disks for High-Data-Rate Recordingxe2x80x9d, Optical Data Storage ""97, Proceedings of SPIE, Vol. 3109, 28 (1997)). In order to solve this problem, a method of equalizing the end-point temperatures of the amorphous and crystalline portions irradiated with beams of the same power level was proposed (e.g., JP 1-149238A). This method requires that the absorptance ratio Ac/Aa is more than 1 (Ac/Aa greater than 1), where Ac is the absorptance of the recording layer when the recording layer is crystalline, and Aa is the absorptance of the recording layer when the recording layer is amorphous with respect to a laser light beam of wavelength xcex, in order to compensate the latent heat of fusion in the crystalline portion.
There are two ways of increasing the absolute value of xcex94R, namely, Rc greater than Ra, i.e., a reflectance-decrease-type in which xcex94R is positive, and Rc less than Ra, i.e., a reflectance-increase-type in which xcex94R is negative. In the reflectance decrease-type, Rc can be raised easily, so that the reflectance as the base can be raised, and Ra can be substantially zero. Therefore, this is advantageous in that the contrast of signals can be large. On the other hand, as described above, either one of the following is necessary in order to increase Ac/Aa at the same time: transmitting part of the incident light or allowing light to be absorbed by a portion other than the recording layer. This is disadvantageous in efficiently utilizing the incident light and in the degree of freedom in the optical design. On the other hand, in the reflectance-increase-type, Ac/Aa can be increased at the same time when the absolute value of xcex94R is increased. Therefore, it is not necessary to transmit part of the incident light or to allow light to be absorbed by a portion other than the recording layer. This is advantageous in efficiently utilizing the incident light and in the degree of freedom in the optical design.
Examples of the structure of such a reflectance-increase-type recording medium are as follows: A structure is such that at least five layers of a semitransparent optical interference layer made of Au or the like, a lower protective layer, a recording layer, an upper protective layer and a reflective layer are formed in this order on a substrate, and the absolute value of xcex94R is increased by the reflectance-increase-type technique utilizing the interference effect of light, especially by the optical interference layer (e.g., JP 7-78354A, JP 7-105574A and JP 7-262607A).
In the recording medium provided with a plurality of information layers as described above, information is recorded on/reproduced from the second information layer positioned farther from the laser light incident side with light that has passed through the first information layer positioned nearer. Therefore, a high transmittance is required for the first information layer, and a high recording sensitivity and a high reflectance are required for the second information layer.
To tackle these problems, the following disk structures were proposed: a reflectance-decrease-type in which no reflective layer is included in the first information layers; and a reflectance-increase-type in which a semitransparent layer is provided in the light incident side in the second information layer. These structures provide high |xcex94R| and Ac /Aa of the first information layer and the second information layer, a high transmittance of the first information layer and a high sensitivity and a high reflectance of the second information layer, and thus good recording/reproducing characteristics are obtained.
However, there are problems as follows. The calculation of the inventors of the present invention concluded that for the first information layer, the reflectance-increase-type recording medium is more advantageous in the optical design in terms of increasing |xcex94R| and Ac /Aa at the same time as described above. However, in the conventional reflectance-increase-type recording medium, the recording layer and/or the reflective layer are thick, so that the transmittance is nearly zero. There is no report that a new reflectance-increase-type recording medium having a high transmittance has been developed and it is possible and effective to use the new medium as the first information layer of a multilayered recording medium.
Furthermore, it is almost impossible to increase the density higher than the current level in recording/reproduction with red laser light. Therefore, there have been efforts to improve the recording density with a smaller beam spot, using blue laser light as a new approach for increasing the density. A problem here is that the optical constants of the recording layer, namely, the refractive index n and the extinction coefficient k, depend on the wavelength. For example, Gexe2x80x94Sbxe2x80x94Te, which is a typical recording material, has larger n and k in the crystalline state than in the amorphous state in the red light wavelength range, whereas it has a larger n in the amorphous state than in the crystalline state in the blue light wavelength range, which is opposite to the former case. Thus, for example, in the above-described structure, it is difficult to achieve large |xcex94R| and Ac /Aa and large transmittance at the same time, especially in the first information layer. Not only in the blue light wavelength range, but also when other materials are used and n in the amorphous state is larger than that in the crystalline state, the same problem is caused.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an optical information recording medium including a plurality of information layers having high C/N ratios and erasure ratios in high density and high linear velocity overwriting. It is another object of the present invention to provide a method for producing such an optical information recording medium, and a method and an apparatus for recording/reproducing information on the optical information recording medium.
In order to achieve the above objects, an optical information recording medium of a preferable embodiment of the present invention includes a first information layer, a separating layer, a second information layer and a second substrate in this order on a first substrate. The first information layer includes a multilayered thin film including a lower protective layer, a recording layer that changes reversibly between at least two different states detectable optically by irradiation of light beams, and an upper protective layer in this order from the side near the first substrate. The reflectance in a region (mark region) where a mark is formed by focusing the light beams incident from the first substrate side on the first information layer is higher than that in a region (space region) where no mark is formed. When the light beams incident from the first substrate side are focused on the second information layer, the ratio of the light beams that pass through the first information layer and reach the second information layer is not less than 40%.
In the above optical information recording medium, it is preferable that the region where a mark is formed is amorphous, and the region where no mark is formed is crystalline. Furthermore, it is preferable that the wavelength of the light beams is not more than 500 nm. Furthermore it is preferable that the first information layer further includes a reflective layer on the upper protective layer on the side of the separating layer. Furthermore, it is preferable that the first information layer further includes a transmittance improving layer on the reflective layer on the side of the separating layer. Furthermore, it is preferable that the first information layer further includes an interface layer at at least one interface selected from the group consisting of the interface between the lower protective layer and the recording layer and the interface between the recording layer and the upper protective layer.
Furthermore, in the above optical information medium, it is preferable that the thickness of the recording layer is from 3 nm to 10 nm. Furthermore, it is preferable that the recording layer contains at least Ge, Sb and Te. Furthermore, it is preferable that 0.10xe2x89xa6xxe2x89xa60.50 and 0.40xe2x89xa6zxe2x89xa60.60 are satisfied, when the atom ratio of Ge, Sb and Te contained in the recording layer is expressed by x:y:z (x+y+z=1).
In order to achieve the above objects, a method of a preferable embodiment of the present invention for producing an optical information recording medium including a first information layer, a separating layer, a second information layer and a second substrate in this order on a first substrate, wherein the first information layer includes a multilayered thin film including a lower protective layer, a recording layer that changes reversibly between at least two different states detectable optically by irradiation of light beams, and an upper protective layer in this order from the side near the first substrate, the reflectance in a region where a mark is formed by focusing the light beams incident from the first substrate side on the first information layer is higher than that in a region where no mark is formed, and when the light beams incident from the first substrate side are focused on the second information layer, the ratio of the light beams that pass through the first information layer and reach the second information layer is not less than 40% includes a film-formation process of laminating the first information layer on the first substrate and the second information layer on the second substrate; an initialization process of putting the first information layer and the second information layer in an initial recordable state; and a tight attachment process of attaching the first substrate and the second substrate via the separating layer in such a manner that the first information layer faces the second information layer. In the film-formation process, the recording layer is formed in an atmosphere including a rare gas and nitrogen as essential components.
In order to achieve the above objects, a method of a preferable embodiment of the present invention for recording/reproducing an optical information recording medium including a first information layer, a separating layer, a second information layer and a second substrate in this order on a first substrate, wherein the first information layer includes a multilayered thin film including a lower protective layer, a recording layer that changes reversibly between at least two different states detectable optically by irradiation of light beams, and an upper protective layer in this order from the side near the first substrate, the reflectance in a region where a mark is formed by focusing the light beams incident from the first substrate side on the first information layer is higher than that in a region where no mark is formed, and when the light beams incident from the first substrate side are focused on the second information layer, the ratio of the light beams that pass through the first information layer and reach the second information layer is not less than 40% is characterized in that the wavelength of the light beams is not more than 500 nm, and information is recorded and reproduced by forming and detecting the mark on the first information layer and the second information layer with the light beams incident from the first substrate side.
In order to achieve the above objects, an apparatus of a preferable embodiment of the present invention for recording/reproducing an optical information recording medium including a first information layer, a separating layer, a second information layer and a second substrate in this order on a first substrate, wherein the first information layer includes a multilayered thin film including a lower protective layer, a recording layer that changes reversibly between at least two different states detectable optically by irradiation of light beams, and an upper protective layer in this order from the side near the first substrate, the reflectance in a region where a mark is formed by focusing the light beams incident from the first substrate side on the first information layer is higher than that in a region where no mark is formed, and when the light beams incident from the first substrate side are focused on the second information layer, the ratio of the light beams that pass through the first information layer and reach the second information layer is not less than 40% includes a light source for generating the light beams with a wavelength of not more than 500 nm; and layer recognition means and layer switching means for forming and detecting the mark on the first information layer and the second information with the light beams incident from the first substrate side.
Thus, the present invention can provide an optical information recording medium provided with a plurality of information layers having a high C/N ratio and a high erasure ratio in high density and high linear velocity overwriting.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.